1. Field of the Invention
This invention relates to circuitry for use in a hearing aid and other applications, having very low current consumption and having gain control cicuitry which is readily controlled to obtain optimum desired performance characteristics, which is highly reliable and which is manufacturable at low cost.
2. Background of the Prior Art
The Killion U.S. Pat. No. 4,170,720, issued Oct. 9, 1979 discloses a high fidelity hearing aid which is particularly advantageous for users having a mild to moderate loss of normal sensitivity for low intensity sounds, but who can readily hear sounds above a certain intensity level. It is operative to amplify sound at low sound intensity levels while operating without substantial amplification at sound intensities above a certain level and it includes a number of highly desirable features for obtaining a high dynamic range and for minimizing distortion. In one embodiment, a control voltage is developed by a peak detector connected to the output of an amplifier which responds to signal levels and which has a logarithmic response characteristic. The control voltage is developed at an ungrounded end of a capacitor which is connected to one end terminal of a potentimeter having an opposite end terminal connected through a resistor to supply potential, the potentiometer being coupled a DC buffer amplifier to a voltage controlled resistance which is connected in circuit with a capacitor in the feedback path of a signal amplifier to control gain, the value of the capacitor being selectable to obtain a level dependent variation in frequency response. An optional FET may be connected in shunt relation to a microphone output to limit the signal amplitude at the input of the signal amplifier.
Hearing aid technology and considerations relating thereto are discussed in a paper of Mead C. Killion and Tom W. Tillman, in the Journal of Speech and Hearing Research, Volume 25, 15-25, March 1882 and in chapter 3 at pages 45-77 of Volume I of a Handbook of Hearing Aid Amplification, 1988, chapter 3 being entitled Principles of High Fidelity Hearing Aid Amplification, by Mead C. Killion, and containing a discussion of considertions relating to the Killion U.S. Pat. No. 4,170,720.
The hearing aid as disclosed in U.S. Pat. No. 4,170,720 is highly advantageous but the circuits as disclosed require a supply voltage of at least about 1.4 volts, thereby requiring a cell such as a silver cell which is not now available at reasonable cost. The circuits also use discrete components and are otherwise more expensive to manufacture than would be desirable.
Other hearing aids as proposed in the prior art would have other disadvantages and, so far as is known, no hearing aids have been used or proposed which would have the advantageous operational features disclosed in the Killion patent and which would be operable with low supply voltages while being manufacturable at reasonable cost.